Sap flow evidence of chilling injury and recovery in mangroves following a spring cold spell

Mangroves are periodically influenced in negative ways by non-freezing temperatures across their global sub-tropical range. However, physiological and morphological evidence of chilling influences to non-freezing chilling events has not been measured in field settings. In this study, we measured sap flow (J_s) during such a chilling (but non-freezing) event in southern China and documented the reductions in J_s and the recovery that ensued. We calculated tree water use (TWU) from J_s measurements taken from thermal dissipation sap flow sensors on two mangrove species (Sonneratia apetala and S. caseolaris). This chilling event significantly injured the mangrove trees in the form of leaf scorch and massive defoliation. Diurnal variations of stem J_s of both species were altered significantly after chilling. On the day of the chilling event, J_s of S. caseolaris was reduced from the daily maximum of 44.1 g H₂O m⁻² s⁻¹ to 0 immediately after chilling, which lasted throughout the remainder of the day. In contrast, S. apetala showed a certain low-temperature tolerance, while still maintaining an adequate transpiration rate after chilling, indicative of a more resilient hydraulic transport system to low temperatures. The sap flow data collected revealed substantial evidence for acute water conservation during low-temperature events, perhaps ameliorating low-temperature damage. Hence, the responses of some mangrove species with high sensitivity to low, but non-freezing, temperature (such as S. caseolaris) may indicate that mangroves possess adaptive whole-tree strategies to cold temperature.